Both professional and amateur golfers use multi-piece, solid golf balls today. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The cover surrounds the inner core and may be made of a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas.
Three-piece, four-piece, and even five-piece balls have become more popular over the years. More golfers are playing with these multi-piece balls for several reasons including new manufacturing technologies, lower material costs, and desirable ball playing performance properties. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a single or multi-layered cover to provide a final ball assembly. Different materials are used in these golf ball constructions to impart specific properties and playing features to the ball.
For instance, in recent years, there has been high interest in using polyurethane compositions to make golf ball covers. Generally, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with hydroxyl-terminated and amine curing agents.
In Sullivan et al., U.S. Pat. No. 5,971,870, thermoplastic or thermosetting polyurethanes and ionomers are described as being suitable materials for making outer cover and any inner cover layer. The cover layers can be formed over the cores by injection-molding, compression molding, casting or other conventional molding techniques. Preferably, each cover layer is separately formed. In one embodiment, the inner cover layer is first injection molded over the core in a cavity mold, subsequently any intermediate cover layers are injection molded over the inner cover layer in a cavity mold, and finally the outer cover layer is injection molded over the intermediate cover layers in a dimpled cavity mold.
In Sullivan et al., U.S. Pat. No. 7,131,915, the outer cover can be made from a polyurethane composition and various aliphatic and aromatic diisocyanates are described as being suitable for making the polyurethanes. Depending on the type of curing agent used, the polyurethane composition may be thermoplastic or thermoset in nature. Sullivan '915 further discloses that compositions for the intermediate cover layer and inner cover layer may be selected from the same class of materials as used for the outer cover layer. In other embodiments, ionomers such as HNPs, can be used to form the intermediate and inner cover layers. The castable, reactive liquid used to form the urethane elastomer material can be applied over the core using a variety of techniques such as spraying, dipping, spin coating, or flow coating methods.
As discussed above, both thermoplastic and thermosetting polyurethanes can be used to form golf ball covers. Thermoplastic polyurethanes have minimal cross-linking; any bonding in the polymer network is primarily through hydrogen bonding or other physical mechanism. Because of their lower level of cross-linking, thermoplastic polyurethanes are relatively flexible. The cross-linking bonds in thermoplastic polyurethanes can be reversibly broken by increasing temperature such as during molding or extrusion. That is, the thermoplastic material softens when exposed to heat and returns to its original condition when cooled. On the other hand, thermoset polyurethanes become irreversibly set when they are cured. The cross-linking bonds are irreversibly set and are not broken when exposed to heat. Thus, thermoset polyurethanes typically have a high level of cross-linking and are relatively rigid.
One advantage with using thermoplastic polyurethane, urea and/or hybrid (TPU) compositions to form golf ball covers is that they have good processability. The resulting thermoplastic materials generally have good melt-flow properties and different molding methods may be used to form the covers. Accordingly, thermoplastic polyurethanes, urea and/or hybrid have been used for years, especially in golf ball covers.
Unfortunately, there are known drawbacks associated with using TPU materials, such as being less durable and less tough than other polymers. In this regard, a resulting thermoplastic polyurethane golf ball cover may not have high mechanical strength, impact durability, and cut and scuff (groove shear)-resistance.
Thus, manufacturers have tried treating thermoplastic polyurethanes in order to enhance the durability and strength of the polymer. For example, an isocyanate may be compounded into a masterbatch and then the masterbatch may be added to the thermoplastic polyurethane composition prior to molding. In another example, the molded thermoplastic polyurethane cover may be dipped into an isocyanate solution. Treating the thermoplastic polyurethane material with isocyanates helps improve the physical properties such as mechanical strength, impact durability, and cut and scuff (groove shear)-resistance of the material. In some cases, the physical properties may not only increase, but they may actually increase beyond the values of the non-refined material.
For example, Kennedy, III, U.S. Pat. No. 8,920,264 and Matroni, U.S. Pat. No. 9,119,990 disclose isocyanate dipping methods, whereby a golf ball having a thermoplastic polyurethane cover is treated with a solution of isocyanate. The isocyanate solution can contain a solvent, for example, acetone or methyl ethyl ketone (MEK), at least one isocyanate compound, and a catalyst. The ball is soaked in the isocyanate solution and this causes the isocyanate compound to permeate the cover. The isocyanate compound cross-links the thermoplastic polyurethane cover material, and this improves the physical properties of the cover such as durability and scuff-resistance.
Manufacturers have also tried applying one or more coating layers about a TPU cover layer in order to improve golf ball properties. In one approach, differing relative proportions of isocyanate functional groups in each of the TPU cover layer and the coating layer enabled the coating layer to react with the TPU cover layer.
However, such approaches require additional processing steps which can be time-consuming and therefore reduce efficiency as well as increase manufacturing costs. Accordingly, there is a need for golf balls incorporating thermoplastic polyurethane, urea and/or polyurethane-urea hybrid compositions that are reliably durable without the need for additional treatments and/or coating layers. The golf balls of the present invention and methods for making same address and solve this need without sacrificing good physical and playing performance properties.